Technically, the Steam Machine supports HDMI 2.1. However, Valve and AMD are not allowed to offer an open-source driver for it.

The HDMI Forum, responsible for the HDMI specification, continues to stonewall open source. Valve’s Steam Machine theoretically supports HDMI 2.1, but the mini-PC is software-limited to HDMI 2.0. As a result, more than 60 frames per second at 4K resolution are only possible with limitations.